This invention relates to improvements in a rolling element such as a cam follower and a rolling bearing, more particularly to a rolling element including constituting members formed of an iron-based material and a diamond-like carbon (DLC) material exhibiting a highly excellent low friction characteristics in presence of a specified lubricating oil or a lubricant, and to a lubricating oil composition for largely improving a friction characteristics at a sliding surface of the iron-based material and the DLC material.
Global environmental problems such as global warming and ozone layer destruction are coming to the fore. As it has been said that the global warming is significantly affected by CO2 emission, the reduction of CO2 emission, notably the setting of CO2 emission standards, has become a big concern to each country. One of challenges to reduce CO2 emission is to improve vehicle fuel efficiency that depends on the performance of engine sliding members and a lubricating oil applied thereto. There are the following approaches to improving vehicle fuel efficiency: (1) lowering the viscosity of the lubricating oil, thereby reducing viscous resistance in hydrodynamic lubrication regions and agitation resistance in the engine; and (2) to adding a suitable friction modifier and other additives into the lubricating oil so as to reduce friction losses under the conditions of mixed lubrication and boundary lubrication.
In view of the foregoing, many studies have been made on various friction modifiers including organomolybdenum compounds, such as molybdenum dithiocarbamate (MoDTC) and molybdenum dithiophosphate (MoDTP). As a result of the studies, it has been found that the lubricating oil containing an organomolybdenum compound successfully produces a friction reducing effect on the steel sliding members in the early stages of use.